The present invention relates to the Internet and other packet-based networks and more particularly to methods for wireless access to packet-based networks by mobile devices.
Support for wireless access between a correspondent node and a mobile device over the Internet is outlined in an Internet Engineering Task Force (IETF) proposal entitled xe2x80x9cIP Mobility Support,xe2x80x9d C. E. Perkinsxe2x80x94Editor, Request for Comments 2002 (October, 1996; hereinafter xe2x80x9cMobile IPxe2x80x9d). By utilizing Mobile IP, each mobile device is always identified by a fixed home address and associated home agent, regardless of its point of attachment to the Internet. Packets sent to a mobile device, from a correspondent node, are directed to the home agent. If the mobile device is away from home, the home agent forwards packets within an IP-in-IP tunnel to an assigned care-of address registered with the mobile device. Such a two-legged routing scheme is known as triangular packet routing. Mobile IP provides a reasonably effective framework for macro-mobility; that is, allowing mobile users to roam away from a home network without disrupting a mobile user""s applications. However, Mobile IP does not effectively support micro-mobility, that is, handoffs of a mobile device between base stations, each of which covers only a very small geographic area. This is because each handoff of a mobile device to a base station riot attached or linked via a node hosting the home agent requires the mobile device to notify the home agent of its associated care-of address regarding the mobile device""s new point of attachment. Therefore, the use of Mobile IP results in messaging and signaling delays and inefficient packet delivery paths to the mobile device.
When the mobile device is in its home network (i.e.xe2x80x94the same network in which the mobile device""s home agent is located), packets destined for the mobile device are intercepted by the home agent. The home agent routes the packets as normal IP packets and sent to the Local Area Network to which the mobile device is normally attached. Therefore, Mobile IP does not support any mobility within the local subnet. If a mobile device changes its point of attachment within a local subnet, the change must be managed by either link layer modification techniques, or by broadcasting packets destined to the mobile device to all base stations attached to the local subnet. Managing the link layer may result in unacceptable delays and packet loss while broadcasting packets to all base stations is an inefficient use of bandwidth.
Recently an extension to the Mobile IP protocol emerged in a draft Internet Engineering Task Force (IETF) proposal entitled xe2x80x9cRoute Optimization in Mobile IP,xe2x80x9d C. E. Perkinsxe2x80x94Editor, Internet Draftxe2x80x94Work in Progress (November, 1997). The route optimization extension proposes a means in which packets may be routed from a correspondent node to a mobile device away from home without first being forwarded to a home agent. Route optimization extensions provide a means for the correspondent node to cache a binding associated with the mobile device and then tunnel packets directly to the care-of address indicated in that binding, thereby bypassing the mobile device""s home agent. Utilizing the proposal, packets are forwarded from an old base station foreign agent to a new base station foreign agent to reduce disruption during handoff. However, a mobile device""s care-of address is nonetheless changed each time the mobile device is handed off between base stations. Although route optimization is proposed as a scheme for improvement in micro-mobility, route optimization still requires undesirable notifications to the home agent and correspondent node for each handoff of the mobile device. Such frequent notification not only increases the amount of control traffic generated, but also places an unnecessary processing burden upon a fixed host which may be providing services to hundreds of fixed and mobile hosts. Until notification of a handoff is completed to the home agent and correspondent node, packets destined for the mobile device are forwarded from the old base base station foreign agent to the new base station foreign agent. During the required round trip messaging time between the home agent and the correspondent node, packets follow an inefficient delivery path resulting in disruption to user traffic.
Local mobility within a subnet is supported by classifying wireless base stations, and the routers used to forward packets to those base stations, within defined domains. Domains are typically defined to incorporate a subnet having a plurality of base stations. Base stations are used by mobile devices to attach to the wired portion of a packet-based network, such as the Internet, and exchange packets thereover with a correspondent node. A home domain is a subnet in which a domain node hosts a home agent of the mobile device. A foreign domain is any domain to which the mobile device is attached, other than the home domain. Packets sent from the correspondent node to the mobile device have a packet destination address corresponding to the mobile device. The mobile device retains this address for the duration of time it is powered up and attached to the Internet via any base station. Therefore, packets destined for the mobile device are always routed to the home domain corresponding to the mobile device.
If the mobile device is attached through a base station included within the home domain, packet tunneling is not required since selected home domain routers and routing capable base stations maintain a routing table entry for the mobile device. The routing table entries are established and updated via path setup schemes to convey packets destined for the mobile device along the proper established path through the domain routers and base stations, regardless of the domain base station through which the mobile device is attached.
However, if the mobile device is attached through a base station included within a foreign domain, the home agent corresponding to the mobile device intercepts packets having the mobile device""s address as a destination address. A care-of address is assigned to the mobile device for the duration of time it is powered up and attached to the Internet via any base station included within the instant foreign domain. The home agent tunnels packets destined for the mobile device to the care-of address for the instant foreign domain. A singular care-of address is used when the mobile device is attached through any base station included within the instant foreign domain, since host based routing in the instant foreign domain is maintained and updated for router and base station routing table entries via path setup messages.
We have observed that mobility is typically a localized phenomenon; that is, the majority of handoffs from one base station to another occur when both the new and old base stations are incorporated within the same subnet. Therefore, for the majority of mobile device handoffs, local routing table entries in selected routers within the domain are updated, but the mobile device address and/or care-of address utilized remain the same. As a result of this observation and the application of the present invention as a mobility solution, handoff notifications to nodes outside of the local domain or subnet, such as to the home agent and the correspondent node, are substantially minimized, making the majority of mobile device handoffs between base stations transparent to the home agent and the correspondent node. In contrast, the aforementioned route optimization extension to Mobile IP not only requires notification to the home agent and the correspondent node for each handoff of a mobile device between base stations, but also requires the assignment of a new care-of address for each handoff outside of the home network, whether or not the base stations involved are both incorporated within the same subnet.